You are in might work land here. Lots of variables Not the least of which is all bets are off if anyone compacts the folders.

But try this.

Install this add-on https://freeshell.de//~kaosmos/index-en.html#recDelMsg

Place Thunderbird in offline mode. File menu > Offline

use the entry the add-on placed in the right click context menu to undelete messages.

Install this add-on https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Create a folder in Local folders and first. export the messages that have been undeleted as an mboc file to local storage in the file system Or EML if you want two backups.

Select the folder you created in Local folder and from the context menu select the import export tools and import that mbox file.

Now take Thunderbird into online mode File menu > offline

It would be normal for the undleted messages to again be deleted on sync, but those in Local folders are no longer associated with the IMAP account and will be fine.

If you wish you can now drag the messages back to the IMAP account and they should sync back to the server. Batches of a few hundred works best.

I am guessing however your customer deleted them in the first place because the IMAP quota is insufficient for their pack rat mail storage requirement. I suggest you establish what quota is available before adding anything to the IMAP account.

Thx for help! I use little diff solution, but core is yours!: 1. install importexporttools 2. go offline 3. connect (import) broken mailbox to local store 4. export content of this mailbox in fs as eml files 5. clean up, if you are out of disk space :) - disconnect recovery store, clean old imap store by compress, 6. go online and import eml files to imap store.